Abstract
Abstract Intraseasonal equatorial Kelvin wave activity (IEKW) at a low frequency in the Pacific is investigated using the Simple Ocean Data Assimilation (SODA) oceanic reanalyses. A vertical and horizontal mode decomposition of SODA variability allows estimation of the Kelvin wave amplitude according to the most energetic baroclinic modes. A wavenumber–frequency analysis is then performed on the time series to derive indices of modulation of the IEKW at various frequency bands. The results indicate that the IEKW activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low-frequency climate variability in the Pacific. Linear model experiments corroborate that part of the modulation of the IEKW is tightly linked to change in oceanic mean state rather than to the low-frequency change of atmospheric equatorial variability.
Highlights
Basin-scale equatorial Pacific variability is dominated by oceanic Kelvin waves at a variety of time scales
It is shown that the Intraseasonal equatorial Kelvin wave activity (IEKW) undergoes significant amplitude changes at decadal time scales for the frequency bands centered around 100, 70, and 50 daysϪ1 for the first baroclinic mode and 100 and 70 daysϪ1 for the second baroclinic mode
Regarding intraseasonal atmospheric variability not as the external stochastic forcing of the El Niño–Southern Oscillation (ENSO) cycle (Eisenman et al 2005), the IEKW modulation might result from rectification processes involving the change in mean state, as for the ENSO modulation (Dewitte et al 2007) or from change in intraseasonal atmospheric variability characteristics (Teng and Wang 2003; Jones and Carvalho 2006)
Summary
Basin-scale equatorial Pacific variability is dominated by oceanic Kelvin waves at a variety of time scales. There are the following three results: 1) there is a low-frequency modulation of highfrequency variability; 2) this modulation is not related in a straightforward manner to ENSO activity and modulation as measured by the scale-averaged wavelet power over the [2–7]-yr band (cf Fig. 3a); and 3) for a particular frequency band (hereafter referred to as the N3VAR index), the first- and second-baroclinic-mode IEKW activities are not necessarily correlated.
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